This disclosure relates to coating compositions and methods for their application. More specifically, it relates to anti-corrosive coating compositions for application to metal substrates.
The most effective, known and long-popular anti-corrosive coatings for application to metal and metal alloy substrates are based on chromate conversion coatings (CCC) and strontium chromate pigmented primers. However, chromates have been found to be carcinogenic, and their use is being phased out, in some places by legislation. Consequently, relatively recent research has involved the exploration of novel corrosion inhibition agents. As a result, various alternative systems with lower toxicity have been investigated, with one such system being hybrid materials based on a modified silica sol-gel process.
The sol-gel process is a well-known, versatile, water based process for making ceramic and glass materials. In general terms, it involves the transition of a system from a liquid “sol” (mostly colloidal) phase into a solid “gel” phase. As applied to silica materials, an aqueous solution of an organosilane such as tetramethoxysilane is formed which, by means of hydrolysis and condensation reactions, forms a new phase, a sol, made up of solid particles of a diameter of a few hundred nanometers suspended in the aqueous liquid phase. Further condensation of the particles forms another new phase, a gel, in which solid macromolecules are immersed in the liquid. All reactions take place at or close to room temperatures, making it possible to incorporate a variety of additives into the gel. Substrates may be dipped into or sprayed with the gel.
In order to provide protective, anticorrosive coatings for metal substrates, however, such silica sol-gel coatings must be rendered impervious and hardened. This can be done by incorporating functional groups such as epoxy groups into the silica sol-gel composition, and then cross-linking (curing) the composition. Epoxy groups may be introduced into the macromolecular composition, for example, by using as a starting material in the silica sol-gel process an epoxy silane such as 3-glycidyloxypropyl trimethoxysilane (GPTMS), alone or in admixture with another organosilane. Curing may then be effected by reaction with a diamine, in the well known way of epoxy resin curing.
U.S. Pat. No. 6,929,826, entitled “Self-Assembled Nano-Phase Particle Surface Treatments for Corrosion Protection”, issued Aug. 16, 2005 purports to describe a chromium-free composition and method for treating metallic surfaces. The composition comprises admixture of an alkoxysilane and an epoxyalkoxysilane and water. The composition is mixed and aged. An aliphatic amine and a surfactant are added to the composition after aging and metallic substrates may then be coated with the resulting solution.
The present disclosure seeks to overcome the aforementioned toxicity problems with anti-corrosive coatings for metal substrates while providing protective coatings of satisfactory, even enhanced protective quality, by replacing the standard chromate compositions with cured polysiloxanes, of significantly reduced toxicity.